In a planar antenna, a strip-line is used to feed a radiating element such as a metal patch to excite the radiating element. In the planar antenna, a parallel-plate mode generated by the excitation of the radiating element propagates. In order to suppress the propagation and to improve various characteristics of the antenna, such as radiation efficiency, a plurality of through holes are provided as electrical conduction between a pair of conductive plates. Reliability of the through holes has been insufficient, because the through holes might be broken by temperature change in the environment of usage or due to long use, so that the suppression effect of the propagation of the parallel-plate mode deteriorates.